The brain is subject to the action of numerous amino acids, amines, and neuropeptides. These local neurotransmitters and diffuse neuromodulators functionally reconfigure neuronal circuitry and allow for flexible, yet stable, behavioral output (Marder, 2012). Dysfunction of modulatory and transmitter systems has been implicated in a number of psychiatric and neurological disorders. In order to understand how the healthy brain mediates this dynamic chemical milieu, we must first understand how single neurons mediate the action of many signaling molecules. The proposed work addresses this question in the crustacean stomatogastric ganglion (STG), a small motor circuit containing 26-30 large, identifiable neurons with complex morphologies. All STG neurons are responsive to both GABA (Swensen et al., 2000) and glutamate (Marder & Paupardin- Tritsch, 1978; Cleland & Selverston, 1998). The subcellular distribution of GABA and glutamate ionotropic receptors, and the electrical properties of the compartments in which they function, likely dictates the weights with which these ligands influence the firing properties of single STG neurons. It is plausible tha these neurons differentially co-regulate and exhibit stereotyped spatial distributions of ionotropi receptors, and that these receptor distributions contribute to their unique firing properties. This hypothesis will be addressed using a custom optical system for focal photo-activation and mapping of ionotropic GABA and glutamate responses across the structures of different STG neuron types. These maps will be complemented with mRNA expression studies to determine if sensitivities to these two ligands are co-regulated in a compartment-specific or neuron- wide manner. Lastly, these maps will be used as a framework for understanding the functional relevance of subcellular receptor distributions in the face of a modulatory perturbation. Previous work has shown that dopamine alters glutamate receptor sensitivity in STG neurons (Cleland & Selverston, 1997; Johnson & Harris- Warrick, 1997). Focal photo-activation of glutamate receptors in tandem with spatially and temporally controlled iontophoretic dopamine administration will be used to probe whether this modulatory change in glutamate sensitivity occurs by uniformly or heterogeneously modulating different subcellular glutamate receptor loci across the structure of single neurons.